a) Field of the Invention
The present invention relates to a retrofocus type photographic optical system which is suited for use in an image pickup apparatus adopted for cameras, in particular digital still cameras, portable telephones, portable mobile personal computers and so on.
b) Description of the Prior Art
As conventional examples of retrofocus type photographic optical system consisting of a small number of lens elements, there are known an optical system disclosed by Japanese Patent Kokai Publication No. Sho 57-163212 which consists of five negative, positive, negative, positive and positive lens elements, an optical system disclosed by Japanese Patent Kokai Publication No. Sho 64-61714 which consists of five negative, positive, negative, positive and positive lens elements or four negative, positive, negative and positive lens elements, an optical system disclosed by Japanese Patent Kokai publication No. Hei 2-85816 which consists of five negative, positive, negative, positive and positive lens elements, and an optical system disclosed by Japanese Patent Kokai Publication No. Hei 9-166748 which consists of five negative, positive, negative, positive and positive lens elements.
Though these optical systems consists of relatively small numbers of lens elements, it is difficult to sufficiently thin camera bodies when these optical systems are used.
There is known an optical system which is configured to fold an optical path in order to thin a camera body. Known as conventional examples of such an optical system are optical systems which are disclosed by Japanese Patents Kokai Publication No. Hei 6-107070 and No. Hei 9-211287. The former is configured for use in an on-vehicle camera and performance of the optical system has not attained yet to a level for use in a digital camera having a large number of picture elements. Furthermore, the optical system is configured with no consideration of focusing. Optical performance and others of the latter are unknown since the patent provides no numerical data though it describes folding an optical path with a prism.
A primary object of the present invention is to provide a photographic optical system which comprises, in order from the object side, a front lens unit comprising at least a negative lens element and having negative refractive power as a whole, and a rear lens unit comprising at least two positive lens elements and at least a negative lens element and having positive refractive power as a whole, comprises a reflecting member to fold an optical path and an aperture stop which are disposed in order from the object side between the front lens unit and the rear lens unit, and satisfies the following conditions (1) and (2):
1.5 less than |fF|/f less than 3.5xe2x80x83xe2x80x83(1)
1.6 less than dM/f less than 2.6xe2x80x83xe2x80x83(2)
Another object of the present invention is to provide a photographic optical system which comprises, in order from the object side, a front lens unit comprising at least a negative lens element and having negative refractive power as a whole and a rear lens unit comprising at least a positive lens element and having positive refractive power as a whole to form an image pickup surface, uses a reflecting member to fold an optical path which is disposed between the front lens unit and the rear lens unit, and is configured to be focused by changing a distance along an optical axis between the front lens unit and the image pickup surface.